SMTP application protocol is clear text. Which is why it is unsecure. The entire conversation can be read (in clear text) by another party that eavesdrop on the conversation. The AUTH LOGIN command is to verify the user for sending mail. However, the actual wire data is not encrypted and still send as clear text.

TLS encrypts the wire traffic. This is however more complex ito handshaking and authentication. The UTL_HTTP package support TLS/SSL (https protocol). But beyond that, I'm not aware of other application interface packages in PL/SQL that Oracle supplies, that support TLS.